Recently, organic light-emitting diode (OLED) has become a mostly focused new generation of display products because of its self-emitting characteristics, high-efficiency, wide color region, and wide viewing-angles, etc. The organic material used to form the OLED plays a critical role in developing OLED.
When the organic material in a light-emitting layer of an OLED is electrically activated, the singlet excitons (S1) and the triplet excitons (T1) are generated. According to the self-spin statistics, the ratio of the singlet excitons (S1) to the triplet excitons (T1) is 1:3. According to the light-emitting principles, the materials of the light-emitting layer of the OLED include fluorescence materials and phosphorescent materials, etc.
The fluorescent materials are only able to use 25% of singlet excitons (S1), which can be back to the ground state S0 by a radiative transition. Thus, the maximum external efficiency of the OLED using the fluorescent materials as the light-emitting layer is unable to break such a limitation. The phosphorescent materials are able to use not only the 25% of singlet excitons (S1), but also 75% of the triplet excitons (T1). Thus, theoretically, the quantum efficiency of phosphorescence materials is 100%; and they are superior to the fluorescent materials when they are used in the OLED. However, the phosphorescent materials are usually rare complexes, the material cost is relatively high. Further, the blue phosphorescence materials have always been having issues on the efficiency and the lifespan when they are applied in the OLED.
In 2011, professor Adachi at Kyushu University, Japan, reported the thermally activated delayed fluorescence (TADF) material. Such a material presented a relatively good light-emitting performance. The band gap value of the S1 state and the T1 state of the TADF material is relatively small; and the lifespan of the T1 excitons of the TADF material is relatively long. Under a certain temperature condition, the T1 excitons may have a reverse intersystem crossing (RISC) to achieve the T1→S1 process; and achieve a radiative decay from the S1 state to the ground state S0. Thus, when the TADF material is used as the light-emitting layer in the OLED, the light-emitting efficiency of the OLED may be comparable to that of the OLED using the phosphorescence materials as the light-emitting layer. Further, the TADF material does not need rare metal elements. Thus, the material cost is relatively low.
However, the existing types of TADF materials are limited; and there is a need to develop novel TADF materials with enhanced performance. The disclosed methods and material structures are directed to solve one or more problems set forth above and other problems in the art.